The present invention relates to the write driver circuit for a hard disk drive (HDD). More particularly, it relates to a hard disk drive write head and circuits for controlling the overshoot of the write head drive current to optimize the rise time and fall time and other characteristics of the write to disk operation.
A hard disk drive storage system typically includes one or more rotating disks, or platters having magnitizible material coated on their surfaces. Read/write heads associated with each platter surface move together radially across the head to reach addressable data regions located on concentric circles called tracks. It is now common to have separate read and write heads. The write head is essentially a small coil of wire which stores data by magnetizing small regions of the disk platter along the tracks. A current driven through the write head creates a temporary magnetic field which magnetizes a small region of the disk at the current position of the write head.
The electronic circuitry used to drive current through the write head typically uses an H-bridge as shown in FIG. 1. For example, U.S. Pat. No. 5,638,012, issued to Hashimoto et al. and incorporated herein by reference, uses an H-bridge for a write driver circuit. The purpose of the H-bridge is to allow electric current to be driven through the write head in either direction. When the current is driven in one direction a magnetic field is created with the north pole in one direction, and when the current is driven in the opposite direction, a magnetic field is created with the north pole in the opposite direction. The H-bridge operates to switch the drive current through the head by turning on a pair of transistors to allow current to flow in a path from a supply source to ground. For example, current flows through the write head from Hx to Hy when transistors Y are turned on and transistors {overscore (Y)} are turned off. Similarly, current flows the opposite direction when transistors {overscore (Y)} are turned on and transistors Y are turned off. The tr, tf (rise time, fall time) is the time corresponding to the speed at which current can reverse through the inductive load of the HDD write head.
It is desirable to increase the speed of the change of current flow to increase the amount of data that can be stored on a single track of the HDD platter. A limitation to decreasing the tr, tf is limiting the amount of current overshoot and the ringing period. FIG. 2 illustrates the current waveform for a test input to a typical prior HDD write head. As the switching speed of the current through the head is increased, the current and voltage overshoot, above the steady state value, increases at the head. While some overshoot can be tolerated, too much overshoot in some applications can have deleterious effects, such as write asymmetry, on the head and consequently the overall drive system performnance. For example, increased overshoot will increase the time for the head current to settle to its steady state value, while it is desirable to have the current settle as quickly as possible.
The prior art circuit provided enhanced drive and the accompanying overshoot by adding capacitors 16, 18 as shown in FIG. 1. In this circuit, at the beginning of the current transitions, additional current is passed through the write head from an initial increase in charge at the bases of the lower transistors supplied by capacitors 16, 18. The increase in initial charge at the bases increased the speed of the write current transitions. The prior art was further enhanced with programmable capacitors to control the overshoot.
In the prior art, the added overshoot resulting from the faster write transitions has undesirable effects. In the present invention, a discharge circuit is added to the base of the bottom transistors of the H-bridge to prevent excessive overshoot and ringing while allowing for faster write transitions.
In another embodiment of the present invention, an adjustable overshoot circuit includes a variable delay.